Method of Monitoring Underground Diffusion of Carbon Dioxide

ABSTRACT

An object is to provide a monitoring method for monitoring underground diffusion of carbon dioxide, which is used when injecting carbon dioxide into an underground coal seam to cause it to be adsorbed to the coal seam, and collecting hydrocarbon gases that have been displaced by the carbon dioxide and released from the coal seam, and with which the behavior of carbon dioxide injected into the ground can be continuously measured over a long period of time at a low cost, using a relatively simple device. 
     A plurality of wells including an injection well  3  and a production well  4  extending to underground coal seams (main layer  1  and lower layer  2 ) are formed. When injecting carbon dioxide into the injection well under pressure, and collecting hydrocarbon gases displaced by the carbon dioxide stored in the coal of the coal seams from the production well  4,  high-precision tiltmeters are placed on the bottoms of observation holes drilled in the ground at a plurality of points (A, B, C, D, E and F) between the injection well  3  and the production well  4,  and by checking chronological changes in the inclination angles at these points with the tiltmeters, diffusion of underground carbon dioxide gas is monitored.

TECHNICAL FIELD

This invention relates to a monitoring method used when injecting carbon dioxide into an underground coal seam to cause it to be adsorbed to the coal seam, and collecting hydrocarbon gases that have been displaced by the carbon dioxide and released from the coal seam, and more particularly to a monitoring method of underground diffusion of carbon dioxide for efficiently collecting hydrocarbon gases by monitoring the behavior of carbon dioxide injected into the coal seam.

BACKGROUND ART

Generally, coal can adsorb gases due to its microscopic pores. Thus, underground coal seams, which comprise coal, contain huge amounts of hydrocarbon gases such as methane gas.

Further, coal can adsorb a several times larger amount of carbon dioxide than methane. By displacing e.g. methane gas in coal with carbon dioxide, it is possible to efficiently and stably store carbon dioxide in coal, and collect methane gas which has been displaced by carbon dioxide as a clean energy source.

There exist technologies for commercially collecting methane gas in coal seams as fuel gases or material gases. (e.g. Patent document 1).

In order to collect hydrocarbon gases such as methane gas from a coal seam, hydrocarbon gases retained in the coal seam are replaced by injected carbon dioxide gas and collected. This is typically carried out as follows.

That is, carbon dioxide gas is injected into the coal seam through a well open to the ground surface. Since the coal seam can adsorb carbon dioxide by an amount e.g. two to several times larger than that of methane, carbon dioxide gas is preferentially adsorbed to the coal surface, so that hydrocarbon gases such as methane gas that have been adsorbed to the coal are released.

Using this mechanism of carbon dioxide-methane displacement, it is possible to collect e.g. methane gas that is present in a large amount in a coal seam from another well as fuel gas.

Coal seams used for this purpose may be deep ones, from which it is difficult to actually mine coal, or low-quality and thus less economically favorable coal seams. Especially in view of the fact that carbon dioxide is one of greenhouse gases, of which discharge control is being strengthened today, the abovementioned technology is considered to be an excellent technology for recycling resources because it can stably store carbon dioxide and can effectively collect natural gases that have been displaced by carbon dioxide.

When storing carbon dioxide gas in a coal seam, and collecting natural gases including methane gas that have been displaced by carbon dioxide in the coal seam, it is known to determine the kinds and concentrations of any beneficial gases in the coal seam by means of a Raman probe and a Raman scattered light analyzer placed in a hollow pipe buried in the ground (Patent document 2).

As means for checking the physical properties and geological structures of other ordinary strata and rock beds, there are also known seismic survey, tomography (elastic wave measurement, specific resistance measurement and electromagnetic wave measurement), geophysical logging (neutron logging, phonometry and density logging).

Patent document 1: JP patent publication 2004-3326A Patent document 2: JP patent publication 2004-309143A

DISCLOSURE OF THE INVENTION Object of the Invention

But because these conventional methods for studying geological structures are all developed for searching oil and coal fields, while they can be advantageously used to measure physical data at a specific point of time, they are not suitable to repeatedly measure changes in data for a long period of time. For such repeated measurements, large-scale equipments and a large cost for measurement were necessary.

Also, with conventional gas monitoring methods, while it is possible to evaluate the gas composition in the ground, the information thus obtained does not reflect the behavior of carbon dioxide injected into the ground. There was therefore no way of easily knowing the behavior of carbon dioxide in the ground.

An object of the present invention is therefore to provide a monitoring method which is free of the above problems and with which the behavior of carbon dioxide injected into the ground can be continuously measured for a long period of time at a low cost, using a relatively simple device.

Means to Achieve the Object

In order to achieve this object, the present invention provides a method of monitoring diffusion of carbon dioxide into the ground, comprising providing at least one injection well and at least one production well which both lead to an underground coal seam, providing a plurality of tiltmeters at intervals in the ground above the coal seam and between the injection well and the production well, injecting under pressure carbon dioxide gas into the injection well to allow the carbon dioxide gas to diffuse into the coal seam, producing hydrocarbon gases in the coal seam which have been displaced by the carbon dioxide gas that has diffused into the coal seam from the production well, and simultaneously monitoring how the carbon dioxide has diffused into the ground corresponding to the amount of production of the hydrocarbon gases by checking chronological changes in inclination angles as indicated on the tiltmeters.

With this method of monitoring diffusion of carbon dioxide into the ground, when carbon dioxide diffuses into the underground coal seam and is stored in the coal seam, the carbon dioxide is dispersed in the coal seam, thereby developing cracks in the coal seam. The cracks cause sinking or rising of the layer above the coal seam, which in turn causes microscopic inclination of this layer.

Thus, by checking chronological changes in inclination angle with a plurality of tiltmeters that are arranged in the ground above the coal seam at intervals, the inclination angles of the tiltmeters chronologically change with time delays as carbon dioxide reaches the respective points of the coal layer located right under the respective tiltmeters. Thus, it is possible to determine that carbon dioxide has reached the points of the coal seam right under the respective tiltmeters.

The inclination angles indicated on the tiltmeters are considered to be related, to some extent, to the amount of carbon dioxide that has diffused into the coal seam. Thus, by checking the tiltmeters, it is possible to infer, to some extent, the behaviors of gases such as to what points of the coal seam, carbon dioxide has reached now, its scattering speed, and the range in which hydrocarbon gases reach.

In particular, by recording the amount of injected carbon dioxide per unit time, and its injected amount and injected period of time until the inclination angles are measured by the tiltmeters, and further if the positions of the injection well and the tiltmeters are known, it is possible to infer the diffusing speed of carbon dioxide by calculation and also possible to infer how many more hours it will take until underground gases are produced.

In this monitoring method for monitoring underground diffusion of carbon dioxide and the accompanying production of hydrocarbon gases, it is important to measure microscopic inclinations due to cracks as accurately as possible. Thus, in this method, it is preferable to use high-precision tiltmeters that can measure inclination angles in increments of 10⁻⁶ to 10⁻⁹ radians.

Also, since it is preferable to accurately measure rising or sinking of the coal seam for accurate monitoring, the tiltmeters are preferably provided in the ground above a cap rock layer because gases such as carbon dioxide gas cannot easily diffuse through the cap rock layer.

For the same reasons, the tiltmeters are provided in the ground at a depth of 10 to 60 m.

Advantages of the Invention

In the monitoring method of carbon dioxide gas injected into a coal seam according to the present invention, using the phenomenon in which when carbon dioxide is scattered in an underground coal seam, cracks develop in the coal seam, so that the ground thereover sinks or rises, and thus microscopically inclines, chronological changes in inclination angles in the ground are measured with a plurality of tiltmeters. Thus, it is possible to easily and continuously measure the behavior of carbon dioxide injected into the ground over a long period of time.

Further, with this method, because it is possible to measure chronological changes in inclination angles in the ground at relative shallow points, using relatively small and simple measuring devices, the cost for the monitoring is low.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically shows the method of monitoring underground diffusion of carbon dioxide.

FIG. 2 is a graph showing the relationship between the injecting conditions of carbon dioxide and chronological changes in the amount of gases produced.

FIG. 3 is a graph showing chronological changes in the east-west and north-south inclinations at point A.

FIG. 4 is a graph showing chronological changes in the east-west and north-south inclinations at point B.

FIG. 5 is a graph showing chronological changes in the east-west and north-south inclinations at point C.

FIG. 6 is a graph showing chronological changes in the east-west and north-south inclinations at point D.

FIG. 7 is a graph showing chronological changes in the east-west and north-south inclinations at point E.

FIG. 8 is a graph showing chronological changes in the east-west and north-south inclinations at point F.

-   1. Main layer -   2. Lower layer -   3. Injection well -   4. Production well -   5. Liquid carbon dioxide tank -   6. Pressure pump -   7. Evaporator -   8. Carbon dioxide injection pipe -   9 a, 9 b. Sump tank -   10. Gas-liquid separator

BEST MODE FOR EMBODYING THE INVENTION

Now the embodiment of this invention is described with reference to the drawings.

The embodiment is directed to a method of monitoring carbon dioxide gas that has diffused into the ground. This method is used in a system for storing carbon dioxide and producing hydrocarbon gases. As shown in FIG. 1, this system includes two wells comprising an injection well 3 and a production well 4 which lead to a lower layer 2 of two coal seams comprising a main layer 1 and the lower layer 2. In this system, carbon dioxide gas is injected under pressure into injection well 3 to store the carbon dioxide gas in coal or the like in the lower layer 2. The carbon dioxide gas thus displaces hydrocarbon gases including methane gas, which are released and collected from the production well 4.

In this production system, in the ground above the coal seams comprising the main layer 1 and the lower layer 2 and between the injection well 3, through which carbon dioxide gas is injected, and the production well 4, through which underground gases are collected, observation holes are drilled at six points (A, B, C, D, E and F). At the bottom of each observation hole, a tiltmeter is placed. With this monitoring method, simultaneously when carbon dioxide gas is injected into the injection well 3, chronological changes in the inclination angle are accurately measured by the plurality of tiltmeters to monitor the behavior of the carbon dioxide gas in the ground, i.e. how the carbon dioxide gas has diffused into the ground, and how underground gases are correspondingly produced.

The observation holes drilled at the six points (A, B, C, D, E and F) are 12 m deep at points A, B, C and D and 50 m deep at points E and F. Points A and B, points B and C, and points C and D are spaced apart from each other by about 25 m, about 80 m, and about 80 m, respectively. Points E and F are located right under points B and C, respectively.

Carbon dioxide used in this invention may be one separated and collected from carbon dioxide-containing exhaust gas produced in thermal power plants or produced when fossil fuel is consumed in factories. High-purity carbon dioxide is relatively easily obtainable with an amine method in which carbon dioxide is separated and collected by allowing it to be absorbed into an amine such as monoethanolamine.

If liquid carbon dioxide thus obtained is used, it is fed under pressure from a liquid carbon dioxide tank 5 by means of a pressure pump 6, and heated and evaporated in an evaporator 7 before being introduced into the injection well 3.

Carbon dioxide is injected into the injection well so as to be injected into a carbon dioxide injection pipe 8 leading to the coal seam (lower layer 2) at a predetermined pressure and temperature. Although such injection pressure and temperature vary with the depth of the coal seam (lower layer 2), if the depth of the coal seam is 500 m, the preferable injection pressure and temperature are presumably about 10 MPa and 30° C., respectively. In this case, the injection pressure reaches the supercritical state at around the injection pressure and temperature at the injection point. But at a point spaced several tens of meters from the injection point, the injection pressure and temperature presumably decrease to 5 MPa and 30° C., respectively. At the depth of 1000 m, the injection pressure may presumably be 15 MPa. At the depth of 3000 m, the injection pressure may presumably be 35 MPa.

Carbon dioxide may be injected through a plurality of such injection wells 3. In the initial stage of injection, the injection pressure is kept at a relatively high level to form many cracks in the coal seam by intentionally breaking the coal seam. Also, sand may be mixed into the coal seam to prevent cracks of the coal seam from closing again, thereby allowing diffusion of carbon dioxide into a wide range of the coal seam for a long period of time.

The production well 4 is preferably spaced from the injection well 3 by a sufficient distance such that carbon dioxide introduced through the injection well 3 into the coal seam can be adsorbed to the coal seam. Such a distance is presumably at least several tens of meters. The production well 4 may be spaced from the injection well 3 by the above distance not only in the horizontal direction, but in any other three-dimensional direction.

For example, carbon dioxide may be injected into a deep portion of the coal seam, and underground gases including methane gas may be collected from its portion right over the portion where carbon dioxide is injected. If the coal seam is inclined, carbon oxide may be injected into its deep portion, and underground gases may be collected from its shallow portion horizontally spaced from the deep portion.

Ordinarily, underground gases collected from the production well 4, which contain e.g. vapor, are fed to a gas-liquid separator 10, in which the liquid contents are separated by an ordinary method, and collected into sump tanks 9 a and 9 b. On the other hand, the aliquoted hydrocarbon gases including methane gas are, after optional refining, fed to facilities that need such gases.

The tiltmeters used in this invention are preferably high-precision tiltmeters capable of measuring angles in increments of 10⁻⁶ to 10⁻⁹ radians. But they are not structurally limited.

Known such high-precision tiltmeters include one having a container in which an electrolytic solution is trapped with an air bubble present in the solution. When the air bubble moves according to the gravitational field, the potential fields in the X-Y directions of the electrolytic solution including the air bubble change. Thus, by measuring the changes in potential in the two directions perpendicular to each other, it is possible to determine the inclination in the X-Y plane. Commercially available such tiltmeters include high-precision tiltmeters made by Pinnacle (USA).

The number of the tiltmeters used and the distances therebetween are not particularly limited. According to the size of the underground coal seam used, the geological properties of the coal seam and the layer thereover, the distance between the injection well and the production well, and/or the monitoring accuracy required, the tiltmeters are provided at intervals of about 30 m to several kilometers (e.g. 1 to 6 km).

EMBODIMENT 1

For coal seams that are similar to those shown in FIG. 1, inclinations were actually measured. The coal seams used in this experiment are located in Minami-Oh-Yubari coal mine in Hokkaido, Japan. The injection well 3 and the production well 4 were spaced from each other by 180 m, with the production well 4 drilled at a point where the inclined coal seams are located higher than their point where the injection well 3 was drilled.

Carbon dioxide was continuously injected under the conditions shown in shown in FIG. 1 (injection pressure (MPa) and the amount of injection (t)) for the period from Nov. 9, 2004 to Nov. 29, 2004. The data of the tiltmeters A, B, C, D, E and F are shown in FIGS. 2 to 8.

As will be apparent from these figures, the north-south inclinations at points A, B and C changed about 1 to 2×10⁻⁶ for the period of 2 to 3 days from November 9, i.e. from the start of injection of carbon dioxide. Thereafter, the north-south inclination at point D also increased and decreased. Then, from around November 20, production of hydrocarbon gases mainly comprising methane gas began. The production thereof increased to 100 m³/day or over and 150 m³/day.

Thus, it was discovered that about 10 days after the inclinations in the north-south directions had changed at points A, B and C after the injection of carbon dioxide gas into the injection well 3, production of e.g. methane gas began in the production well.

The rates of change in the inclination angles at points B and C are small compared to that at point A, which is nearer to the injection well. But because it is apparent that by increasing the injection amount of the carbon dioxide, the relationships among the amount of carbon dioxide injected, the distance and the inclination becomes clearer, it is possible to adjust the injection amount and pressure of carbon dioxide according to the geological features and depth at the measuring point. By using the data thus obtained, it is possible to plan the production of e.g. methane gas.

When changes in inclination was observed on the tiltmeters at points B and C, no change in inclination was observed on the tiltmeters provided at points E and F, which were located below a cap lock layer.

This indicates that by providing tiltmeters at locations above the cap rock layer, which lies above the coal seams, the influence of the behavior of carbon dioxide gas is reflected more sharply on the tiltmeters, so that monitoring is possible which allows more accurate planned production. 

1. A method of monitoring diffusion of carbon dioxide into the ground, comprising providing at least one injection well and at least one production well which both lead to an underground coal seam, providing a plurality of tiltmeters at intervals in the ground above the coal seam and between the injection well and the production well, injecting under pressure carbon dioxide gas into the injection well to allow the carbon dioxide gas to diffuse into the coal seam, producing hydrocarbon gases in the coal seam which have been displaced by the carbon dioxide gas that has diffused into the coal seam from the production well, and simultaneously monitoring how the carbon dioxide has diffused into the ground corresponding to the amount of production of the hydrocarbon gases by checking chronological changes in inclinations angles as indicated on the tiltmeters.
 2. The method of monitoring diffusion of carbon dioxide into the ground as recited in claim 1 wherein said tiltmeters can measure inclination angles in increments of 10⁻⁶ to 10⁻⁹ radians.
 3. The method of monitoring diffusion of carbon dioxide into the ground as recited in claim 1 wherein said tiltmeters are provided in the ground above a cap rock layer which lies above the coal seam.
 4. The method of monitoring diffusion of carbon dioxide into the ground as recited in claim 1 wherein said tiltmeters are provided in the ground at a depth of 10 to 60 m.
 5. The method of monitoring diffusion of carbon dioxide into the ground as recited in claim 2 wherein said tiltmeters are provided in the ground above a cap rock layer which lies above the coal seam.
 6. The method of monitoring diffusion of carbon dioxide into the ground as recited in claim 2 wherein said tiltmeters are provided in the ground at a depth of 10 to 60 m. 